In recent years it has been desired in various fields of application to enhance outputs of semiconductor laser elements. To enhance the output of a semiconductor laser element, firstly, it is necessary to suppress catastrophic optical damage (hereinafter abbreviated to COD) that occurs on an end facet of the semiconductor laser element by reducing the light intensity of a waveguide mode in an active layer of the semiconductor laser element. Secondly, it is necessary to improve the temperature characteristic of the semiconductor laser element by efficiently confining injected carriers within the active layer.
FIGS. 3A to 3D are views showing an example of a semiconductor laser element of a conventional technology. The semiconductor laser element 2 in FIGS. 3A to 3D is a semiconductor laser element of a well known separate confinement heterostructure (hereinafter abbreviated to SCH structure), which exhibits an example in which an active layer 21 comprises one quantum well layer 21a and two barrier layers 21b between which the quantum well layer 21a is sandwiched. FIG. 3A is a cross-sectional view of the semiconductor laser element 2, FIG. 3B is a distribution diagram of bandgap widths corresponding to the respective layers of the semiconductor laser element 2, FIG. 3C is a distribution diagram of refractive indices corresponding to the respective layers of the semiconductor laser element 2, and FIG. 3D is a graph showing a waveguide mode of the semiconductor laser element 2. In FIGS. 3B to 3D, the abscissa represents positions in the semiconductor laser element 2 in the direction of thickness.
The semiconductor laser element 2 of the SCH structure is made by epitaxial growth, on a semiconductor substrate (not shown) formed of n-GaAs, of an n-type clad layer (n-AlGaAs) 22, an active layer (single quantum well layer of GaAs/AlGaAs) 21 comprising a quantum well layer 21a and barrier layers 21b, and a p-type clad layer (p-AlGaAs) 23 one on top of another. Note that, in FIGS. 3B to 3D, the n-type clad layer 22 corresponds to a region between T10 and T11, the quantum well layer 21a corresponds to a region between T12 and T13, the barrier layers correspond to regions between T11 and T12 and between T13 and T14, and the p-type clad layer 23 corresponds to a region between T14 and T15.
The semiconductor laser element 2 of the SCH structure is so configured that each of the clad layers 22, 23 for confining carriers itself assumes the role of confining the waveguide mode. Since the structure for confining the carriers is at the same time the structure for determining the waveguide mode, the semiconductor laser element 2 of the SCH structure has a problem that it is not possible to reduce the light intensity of the waveguide mode in the active layer 21 freely. That is, there is a problem that it is difficult to design the semiconductor laser element 2 so as to suppress occurrence of COD on the end facet, which imposes restriction on making the semiconductor laser element 2 operate with high output.
A semiconductor laser element 3 of a decoupled confinement heterostructure (hereinafter abbreviated to DCH structure), which was proposed with the aim of overcoming the problem with the semiconductor laser element 2 of the SCH structure and designing the light intensity of the waveguide mode in the active layer freely, is disclosed, for example, in Japanese Examined Patent Publication JP-B2 3658048.
FIGS. 4A to 4D are views showing a semiconductor laser element 3 of the DCH structure. FIG. 4A is a cross-sectional view showing the semiconductor laser element 3, FIG. 4B is a distribution diagram of bandgap widths corresponding to the respective layers of the semiconductor laser element 3, FIG. 4C is a distribution diagram of refractive indices corresponding to the respective layers of the semiconductor laser element 3, and FIG. 4D is a graph showing a waveguide mode of the semiconductor laser element 3. In FIGS. 4B to 4D, the abscissa represents positions in the semiconductor laser element 3 in the direction of thickness.
The semiconductor laser element 3 of the DCH structure is made by epitaxial growth, on a semiconductor substrate (not shown) formed of n-GaAs, of an n-type clad layer (n-AlGaAs) 36, an n-type waveguide layer (n-AlGaAs) 34, an n-type carrier blocking layer (n-AlGaAs) 32, an active layer (single quantum well layer of GaAs/AlGaAs) 31 comprising a quantum well layer 31a and barrier layers 31b, a p-type carrier blocking layer (n-AlGaAs) 33, a p-type waveguide layer (n-AlGaAs) 35, and a p-type clad layer (p-AlGaAs) 37 one on top of another. Note that, in FIGS. 4B to 4D, the n-type clad layer 36 corresponds to a region between T20 and T21, the n-type waveguide layer 34 corresponds to a region between T21 and T22, the n-type carrier blocking layer 32 corresponds to a region between T22 and T23, the active layer 31 corresponds to a region between T23 and T24, the p-type carrier blocking layer 33 corresponds to a region between T24 and T25, the p-type waveguide layer 35 corresponds to a region between T25 and T26, and the p-type clad layer 37 corresponds to a region between T26 and T27.
In the semiconductor laser element 3 of the DCH structure, as shown in FIG. 4B, carriers, having overflowed beyond each of the carrier blocking layers 32 and 33 due to temperature increase of the active layer resulting from injection of high driving current, distribute in each of the waveguide layers 34 and 35 having a bandgap width smaller than that of each of the carrier blocking layers 32 and 33. The carriers, once overflowed, are inhibited from diffusing in the opposite direction back into the active layer 31 by the high potential barrier of each of the carrier blocking layers 32 and 33. Accordingly, there is a problem that the efficiency of carrier confinement within the active layer 31 tends to decrease.
Especially, when the semiconductor laser element 3 is formed of a group III-V compound semiconductor such as AlGaAs, InGaAs and InGaAsP, due to a difference in effective mass between electrons and holes, that is, since electrons are smaller in effective mass than holes, in the active layer 31, the carrier density of electrons becomes relatively higher in comparison with the carrier density of holes. In other words, with respect to the carriers overflowing from the inside of the active layer 31, electrons are greater in number than holes. Therefore, with the semiconductor laser element 3, there is a problem that the efficiency of electron confinement tends to decrease.